1. Field of the Invention
The present invention is directed to systems and methods for monitoring offshore platform supports, and, in certain particular aspects, to systems and methods for monitoring rack path differential of jack-up platform legs and loading on the legs.
2. Description of Related Art
Offshore platforms and rigs for well operations, some of which are called “jack-up platforms,” are floating vessels with legs driven by jacking units extending down and contacting the sea floor. A “jackup” rig or platform is any self-elevating offshore platform or deck with one or more legs, each leg consisting of one or more chords, used for any of a variety of operations, including, but not limited to, drilling, production, workover, or other offshore operations or work, which has the ability of being supported on jackable leg(s) to the seafloor, optionally with the capability of relocating from one offshore location to another by lowering to an afloat position, being moved to a new offshore position, and raising itself again to an elevated position. Such “off-shore” rigs and platforms are well-known and typically include a drilling rig or “hull” generally mounted on multiple legs, e.g. three or more legs, each provided with a base forming a supporting foot (“spud can” of the leg). In certain systems, each has one or more chords structurally connected to each other forming one unit (a “leg”). Legs with one to four chords are now in use. Each chord has one or two (opposite to each other) racks. Each rack is driven by one or more pinions. The rig or platform is positioned at a desired site, and then raised to an operational height above the sea. Motorized displacement (“jacking”) of the rig along each of its legs raises the rig. Each a leg can be raised independently of the other legs to a certain extent, so that an operator can, for example, correct uneven penetration of the legs into the sea bed.
The rig is raised or lowered in relation to each leg by the sets of racks-and-pinions driving each leg, the systems generally with three legs being arranged in each angle or corner of a triangular structure with three metal chords and struts (“chord” designating each “member” of each leg). The jacking system of this type includes at least one elongated rack which is mounted vertically on the exterior side surface of the upright legs and extends substantially through the entire length of the same, and a plurality of cooperating pinions engaged with each of the racks. Each of pinions is driven through a series of reduction gears by means of a respective motor. When the platform is supported by the upright legs on the sea floor, this support is effected by the engagement of the rack with the pinion of the jacking system. In this manner, during offshore operations the load which is composed of the self-weight of the platform and environmental forces such as wind, wave, current and others is placed on the jacking system in engagement of the rack with the pinion.
When the position of a given leg is changed, the pinions relating to this leg are in operation simultaneously, in the same direction and with the same theoretical speed. The speed of linear displacement depends on the load on the leg. When a leg is inclined, the least-loaded chord is raised more quickly than the others by its motors, resulting in an additional increase in “Rack Path Difference” or rack phase differential (or “RPD”) on the leg concerned. The relative position of the rig (hull) and of the legs (geometry of the system) in relation to the sea bed is, in many prior systems, checked in relation to two series of fixed reference points, which are the bottom of the hull of the rig and the top of the “jacking structure” or “jacking house”.
Correct positioning can be inhibited by many factors: e.g. incorrect positioning by an operator; the existence or appearance of major lateral stresses or loads, such as those due to currents, swell and/or wind; uneven embedding of the feet of the legs; heterogeneous or inclined ground; or to an operating fault on a lifting motor or brake. Often it is difficult to determine which factors are involved. Excessive stresses on the structure and particularly on the legs, with the risk of damage to the legs, can lead to rig down time and a significant reduction of the service life of the rig.
“Rack Path Differential” (difference for a given rack path, i.e. geometrical difference in the structure for a given number of rack notches) is a horizontality defect. Normally, when the rig is on site, the legs are lowered until their tips are resting on the sea bed, then the rig is raised out of the water up to its operating position. This involves a certain penetration of the tips into the sea bed, according to the nature of the bed, but normally the legs remain vertical and the only forces acting on the unit remain within the strength limits selected when designing the rig. However, in certain cases, the sea bed may be inclined or uneven, etc., which can cause a horizontal deviation, a deviation of one or more legs in relation to the vertical, which creates a bending moment on the leg or legs concerned. These deviations, if they affect two or three legs, are not necessarily parallel to each other, which can complicate the problem. Such bending causes the load to become unequal on the three chords of the leg concerned, the leg being in a skewed position with respect to its guide. The imbalance can be such that it is no longer possible to move the rig. The rig then has to be lowered again to water level, to a floating state to eliminate the load, the supporting legs are withdrawn from the sea bed over a part of the penetration obtained, and then the jacking operation is recommended. This may possibly be combined, according to the seriousness of the situation, with a slight shift of position in order to avoid the first footprints, although such an operation is not generally recommended, and with a backward and forward movement of the legs such as reaming to correct the deviation. Such action is obviously time consuming and is not always successful. In certain severe situations, a decision has to be made to move the rig from the planned drilling point to another location 50 to 100 meters away, and to recommence the operation, with the same uncertainties. This latter solution is impossible to implement when the rig has to be located alongside a fixed production platform. In such a case, only a margin of a small distance is available for jacking up the rig.
There has long been a need, recognized by the present inventors, for effective and efficient systems and methods for both monitoring loads on rig legs and for monitoring changes in position of the legs.